Not true. Anti-freeze is a magic elixer. It lowers the freezing point of the solution but it also raises the boiling point. The heat exchange factor could be as you say but if he's already prone to overheating, then he could actually overheat quicker if the water starts to boil.
I've never experienced it but was told that the vanes on the water pump can erode over time. On that tid-bit of info I changed the water pump on the Eclipse when I was having overheating issues. It was fine but it was time to replace the timing belt anyway so I didn't get too pissed.
I don't know if the VX has a similar problem but some engines are prone to getting an air bubble right behind the thermostat. That air bubble keeps the thermostat from responding to the water temp & opening. You have to get a special tool (can't remember what it's called) that fits on the opening of the radiator in place of the cap and allows you to have extra anti-freeze above the level of the radiator. When the bubbles work their way out, the anit-freeze flows in to replace.
If you do replace your thermostat, make sure you get the kind that has the blow-by plug. Supposedly if your engine overheats, fluid will leak by enough to save your engine ... in theory. In reality, take a pair of pliers and pull that plug out before you install in in the engine. Also, install it with the hole at the highest level. It should allow any trapped air to flow through and solve that trapped air problem.
Now to qualify EVERYTHING I've said. I'm still having overheating problems with the Eclipse so I may be out in left field. I've isolated the problem down to a relay that controls the fan (it never comes on).